Electronics > Projects, Designs, and Technical Stuff
measuring mA AC using uC
NorthGuy:
--- Quote from: hussamaldean on June 05, 2019, 04:37:34 am ---I will buy 100A current meter which commonly designed for control panels which comes with current transformer
what value of resistance shall I use for 1Amp range,10Amp and 100Amp range ?
--- End quote ---
For 100A, you don't want a big shunt because it'll waste energy. Commonly used values in solar installations are 0.1mOhm, or even 0.05 mOhm. Keep in mind, that at such currents, wires create significant voltage drops, so your ground is likely to be different from the shunt ground. Then you would use an instrumentation amplifier to increase the voltage range to 3V (or whatever your ADC needs).
For 100A AC, a current transformer is a much better idea.
hussamaldean:
--- Quote from: NorthGuy on June 05, 2019, 05:08:13 am ---
--- Quote from: hussamaldean on June 05, 2019, 04:37:34 am ---I will buy 100A current meter which commonly designed for control panels which comes with current transformer
what value of resistance shall I use for 1Amp range,10Amp and 100Amp range ?
--- End quote ---
For 100A, you don't want a big shunt because it'll waste energy. Commonly used values in solar installations are 0.1mOhm, or even 0.05 mOhm. Keep in mind, that at such currents, wires create significant voltage drops, so your ground is likely to be different from the shunt ground. Then you would use an instrumentation amplifier to increase the voltage range to 3V (or whatever your ADC needs).
For 100A AC, a current transformer is a much better idea.
--- End quote ---
what about 1Amp using current transformer, will same 100Amp current transformer work with 1Amp if I change the resistance ?
Kleinstein:
In principle the 100 A current transformer would still work at 1 A. However there can be some extra errors (e.g. due to core hysteresis). Not all current transformers are made the same. I would increase the shunt at the secondary, but not by a factor of 100 to get the same ouput voltage, but maybe only 10 times so that the core would not see so much field.
Depending on the construction one may be able to use more primary turns (e.g. 5 turns with the outer part of the turns to 4 different directions).
A good starting point for a mall DIY current transformer would be a common mode choke (e.g. from an old SMPS / PC supply). For not too high demand these cores are not that different from the ones in current transformers.
hussamaldean:
--- Quote from: Kleinstein on June 05, 2019, 06:50:06 am ---In principle the 100 A current transformer would still work at 1 A. However there can be some extra errors (e.g. due to core hysteresis). Not all current transformers are made the same. I would increase the shunt at the secondary, but not by a factor of 100 to get the same ouput voltage, but maybe only 10 times so that the core would not see so much field.
Depending on the construction one may be able to use more primary turns (e.g. 5 turns with the outer part of the turns to 4 different directions).
A good starting point for a mall DIY current transformer would be a common mode choke (e.g. from an old SMPS / PC supply). For not too high demand these cores are not that different from the ones in current transformers.
--- End quote ---
well, I got myself 100A/100mA current transformer what the values of the resistance shall I use for 1A,10A and 100A ranges ?
regards
David Hess:
--- Quote from: hussamaldean on June 05, 2019, 05:40:12 am ---what about 1Amp using current transformer, will same 100Amp current transformer work with 1Amp if I change the resistance ?
--- End quote ---
Current transformers have a maximum volt*second specification given for their secondary before they saturate so raising the secondary shunt resistance, while proportionally increase the sensitivity, also limits the maximum current before saturation. So changing your 100 amp current transformer into a 1 amp current transformer by increasing the secondary resistance by 100 times means that it will be limited to accurate measurements of up to 1 amp.
You can safely overload a current transformer however it may be necessary to limit the secondary voltage with diodes or something similar and I have sometimes seen this done.
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